Our objective is to investigate the phase separation of protein solutions in normal and sugar-cataract lenses of rats and mice using the technique of laser light scattering spectroscopy. We have discovered that the opacity in cold cataract is the result of light scattering by spatial fluctuations of refractive index formed by interspersed regions of two separated phases of the protein-water mixture in the fiber-cell cytoplasm. It has further been found that the nuclear opacities in the galactosemic cataract of rat lenses, the hereditary cataract of Nakano mice and the hypoglycemic cataract in organ-cultured rat lens are due to this phase separation. In normal rat or mouse lens the phase separation temperature decreases with age. In these galactosemic, hypoglycemic and Nakano cataractous lenses the phase separation temperature starts to increase at a certain stage, and eventually reaches ocular temperature when the in vivo opacity appears. Based on these discoveries we propose to establish the dynamic behaviors of the phase separation in the lenses. Our long-term goal is to understand the physical basis of opacities in human cataract associated with sugar-metabolism defects such as galactosemia, hypoglycemia and diabetis.